How to get started with embeddable scripting?

Question

I am working on a game in C++. I've been told, though, that I should also use an embeddable scripting language like Lua or Angelscript, but to be honest, I have no idea how or why. What advantages would this bring me, over storing all of my data in some sort of text file? How do I get started? I tried to read some Lua examples, but I don't see how it works, or how exactly I am supposed to use it.

Answer 1

First the "why" question:

If you've made reasonable progress so far, you have game scenery where the action happens, and then a kind of GUI with your visible game controls: Maps, compass, hotkeys, chat box, whatever.

If you make the GUI (positions, sizes, settings, defaults, etc) configurable through a configuration file, that's OK for starters. But if you make it controllable by code then you can do many very cool things. Example: Minimize the map when entering a city. Show other player's portraits when in group. Update the map. Display different hot keys in combat. That kinda thing.

Now you can do your code-controlling of your GUI in C/C++ code, but one problem is that whenever you want to change the behavior, even if only a little, you need to recompile the whole dang game client. If you have a billion players, you have to ship them all a new game client. That's a turn-off. Another problem is that there's no way on earth that a player can customize the GUI.

A simple embedded language solves both problem. You can put that kind of code in separate files that get loaded at runtime and can be fiddled with to anyone's heart's content. If you want to update the GUI in some minor way, you can deliver updates of the GUI code separately from the game proper.

As for the how:

The simplest thing to do is to call a (e.g.) Lua "main" routine once for every frame, perhaps passing a bunch of parameters with the latest updatable information, and let that main routine call other functions to do whatever's needed. The thing to be aware of is that your embedded code only gets control for a short time, namely the time between two screen refreshes; so it does a little updating and painting, then it exits again and returns control to your C/C++ main program loop.

Technically, embedding a Lua interpreter in your program is pretty easy. The Lua interpreter has C source code, or there's pre-compiled libraries (DLLs) for Windows. Just link them into your program, initialize once, call the entry point on every iteration of the main frame loop, done.

Answer 2

For some inspiration, check out the Alternate Hard and Soft Layers pattern described on the C2 wiki.

As for my two cents, why embed a scripting language? Some reasons that I've experienced include,

• REPL
• easy string manipulation tools
• leverage the power of loops, macros, and recursion within your data set
• create dynamically generated content
• wrappers to fetch content from the web
• logic to provide default values if data is missing
• unit tests written at the data set level

Answer 3

Scripts are more powerful than storing all of your data in text files. You can assign arbitrary behavior, construct data from other data (e.g., orc captains are orcs with a bit more), and so on.

Scripts allow for faster development and easier maintenance than C++. No compile / edit / link cycle, you can even tweak the scripts while the game is running, and they're easier to update on end users' machines.

As far as the how, one suggestion would be to see how other games do it. For example, TOME, a Roguelike RPG written in C, uses Lua extensively.